N-type semiconductor cubic boron nitride (c-BN) has high resistance against heat (stable up to about 1300.degree. C.), a wide band gap (7 eV) and excellent stability against chemical reactions. N-type c-BN has been attracting strong attention as a promising material of heat resistant semiconductor devices, or optoelectronic devices for emitting the light ranging from ultraviolet to visible light. However, cubic boron nitride is not yielded as a natural resource. Ultra high pressure method synthesizes bulk crystals of c-BN.
Furthermore, vapor phase synthesis method fabricates thin films of c-BN on some substrate. Non-doped c-BN is an insulator with high resistivity. P-type c-BN is obtained by doping with beryllium (Be). Doping with silicon (Si) or sulfur (S) brings about n-type c-BN. Application to semiconductor devices necessitates a pn-Junction. A pn-junction can be fabricated on a c-BN by synthesizing an n-type c-BN crystal on a p-type c-BN crystal as a seed by the ultra high pressure synthesizing method, although it is difficult. Since a pn-Junction can be produced on a c-BN, some applications have been suggested regarding the semiconductor devices of c-BN.
1 Japanese Patent Laying Open No. 4-11688 proposed an application of c-BN to semiconductor devices.
2 Japanese Patent Laying Open No. 3-112177 made an offer of an application of unifying c-BN with diamond.
3 Japanese Patent Laying Open No. 1-259257 also suggested a complex of c-BN and diamond as a semiconductor device.
4 Japanese Patent Laying Open No. 4-29375 proposed an ohmic electrode of c-BN by the same inventors of the present invention.
An ohmic electrode is one of the most important parts in order to produce semiconductor devices. However there was no ohmic electrode of c-BN due to the too short history of c-BN as a semiconductor. Nobody except the Inventors has suggested a method of fabricating ohmic electrodes of c-BN. Only the Inventors of the present invention have suggested an ohmic electrode on a c-BN crystal in the above application. An ohmic electrode was produced with titanium (Tl), zirconium (Zr), hafnium (Hf) or an alloy including Ti, Zr and Hf.
5 Japanese Patent Laying Open No. 4-29376 suggested a method of producing an ohmic electrode with a metal including silicon (Si)or sulfur (S), or an alloy containing silicon (Si) or sulfur (S) by the same inventors.
6 Japanese Patent Laying Open No. 4-29377 made an offer of forming ohmic electrode of c-BN with boron (B), aluminum (Al), gallium (Ga), indium (In) or an alloy of B, Al, Ga or In.
7 Japanese Patent Laying Open No. 4-29378 mentioned a method of producing an ohmic electrode with vanadium (V), niobium (Nb) or tantalum (Ta) or an alloy including V, Nb or Ta.
An ohmic electrode is an electrode formed on a substrate or a film which is in ohmic contact with the substrate or the film crystal. "ohmic" means bilateral current-voltage property unlike diodes or rectifiers. The forward resistance is equal to the rear resistance at an ohmic electrode. Further, an ohmic electrode demands both small forward resistance and small rear resistance in order to minimize the loss of signal power at the electrode.
Besides the inherent resistance at the electrode, it is also desirable to reduce the resistance of the circuits following the electrode in order to produce good devices. Therefore, the ohmic electrode shall preferably adopt a multilayer structure in which more conductive Au or Al layer covers an ohmic contact metal in order to connect the contact metal with circuit patterns with little resistance. The higher layer metal (Au or Al) may be called an extraction electrode, since the layer connects the higher resistive electrode with outer circuit patterns or outer wires.
Prior art from 4 to 7 suggested ohmic electrodes of titanium (Ti), zirconium (Zr), hafnium (Hf), a metal containing Si or S, boron (B), aluminum (Al), gallium (Ga), indium (In), vanadium (V), niobium (Nb), tantalum (Ta) or so on. These materials can all form an ohmic contact with cubic boron nitride (c-BN). The contact has the bilateral character. The forward resistance and the rear resistance are nearly equal. Thus the electrodes can be called "ohmic".
However, these prior art did not satisfy the other requirement of ohmic electrodes. The metals or half-metals recommended as ohmic contact materials have resistivities more than ten times as high as that of gold (Au). Simple use of these high resistive metals or semi-metals would raise the resistance of the electrodes. Thus the prior structure adopted the extraction electrode (Au or Al) to reduce the resistance between the circuit patterns and the ohmic electrodes. The deposition of Au on the resistive material is also useful for wire-bonding between the electrodes and the pattern circuit pads. Prevention of oxidization of the ohmic electrodes necessitates the coating with Au.
High heat resistance is also required for the ohmic electrodes in order to apply c-BN to heat-resistant semiconductor devices or optoelectronic devices for emitting the light ranging from ultraviolet rays to visible rays. However it has been noticed that the electrodes suggested by 4 to 7 lack the heat resistance. Titanium (Ti), zirconium (Zr) and hafnium (Hf) are metals with a high melting point among the ohmic electrodes proposed by 4 to 7. Thus someone may suppose that the ohmic electrodes fabricated with the high melting point metals should enjoy high resistance against heat.
However the fact was otherwise to their surprise. The electrode of (c-BN)/(ohmic electrode of Ti, Zr or Hf)/(Au) in strata has a drawback that Ti, Zr or Hf atoms easily diffuse into the Au surface layer by heating at about 300.degree. C. which is far lower than the melting points of the materials. Diffusing through the Au layer, the refractory metal atoms attain the surface of the Au layer and cover the Au layer surface. The segregation of the refractory metal on the Au layer raises the contact resistance of the ohmic electrode. A moderate temperature such as about 300.degree. C. impairs the property of the ohmic electrode by the precipitation of the metal on the extraction layer of Au. These devices are useless even at about 300.degree. C. because of to the increment of resistance by the diffusion of the metal in the Au layer.
Furthermore, even when the devices are operated by a large driving current at room temperature, the ohmic electrode Is heated at a temperature as high as 300.degree. C by the generation of Joule's heat. The heat facilitates the diffusion of the high melting point metals in Au. Thus the drive of the devices even at room temperature degrades the performance of the device by increasing the extraction resistance by the deposition of metals on the top surface of the Au layer.
A purpose of the present invention is to provide an ohmic electrode of c-BN which prevents the refractory metal Ti, Zr or Hf composing the electrode from diffusing in the Au layer till the top surface. Another purpose of this invention is to provide an ohmic electrode immune from the segregation of the high melting point metal on the Au layer. Another object of the invention is to provide a c-BN semiconductor device with high heat resistance by an ohmic electrode endowed with low resistance at high temperature. Still further object of this invention is to provide an optoelectronic device which can emit highly luminous rays from ultraviolet wavelength to visible light wavelength by a big current injection.